1. Field of the Invention
The present invention relates to an image processing method and apparatus, and more specifically, to an image processing method and apparatus for decreasing the degradation of a printed image caused by the relative deviation of formed dot locations resulting from sheet feeding, in a printing apparatus such as an ink jet printer.
2. Description of the Prior Art
A sheet feeding mechanism for feeding a sheet in a printing apparatus such as a printer generally includes two sets of rollers provided on upstream and downstream sides of a print head, respectively, and rotates these rollers to feed the sheet at a predetermined amount as the print head is caused to execute a printing operation. For example, on the upstream side is provided a pair of a roller rotated by driving force and a pinch roller for pressing the sheet against the above roller to generate feeding force, whereas on the downstream is provided a pair of a roller similarly rotated by driving force and a spur for pressing the sheet against the above roller. Then, the sheet is fed under a condition of making a peripheral velocity of the downstream roller slightly faster than that of the upstream roller and of applying an appropriate tension to the sheet between these pairs of rollers. Thus, a printing surface of the sheet on which the print head is caused to execute the printing operation can be kept flat and the sheet can be fed accurately.
As described above, the sheet is relatively accurately fed when the sheet is located between the upstream roller and the downstream roller. At the start or end of sheet feeding, however, when the sheet is fed with its front or back end is disengaged from the corresponding pair of rollers, it cannot be fed so accurately. As a result, dots may not be formed at normal locations in a feeding direction, resulting in a feeding error in which the dots are mutually deviated.
On the other hand, it is common for the printer to output an image on a sheet fed with one of its ends disengaged from the corresponding pair of rollers, as in the printing of a photograph image on an entire space of sheet of a predetermined size.
FIGS. 43A and 43B are views for explaining feeding of the sheet (hereinafter also referred to as a “printing medium”). FIG. 43A shows that the sheet is fed only by upstream rollers at the end of sheet feeding. FIG. 43B shows that the sheet is fed only by downstream rollers at the start of sheet feeding.
As shown in FIG. 43A, an area of the printing medium 2 between a location of a back end thereof at the time when the medium is disengaged from an upstream roller 3A and a location on the medium immediately below a back end of a printing width of the print head 1 as observed at that time (this area is hereinafter referred to as a “back end area”) is fed only by the downstream roller 3B. Then, the back end area of the medium faces (the printing width of) the print head 1 while being fed. In this state, the sheet remains flat due to its rigidity. However, since the sheet is disengaged from the roller 3A and the corresponding pinch roller, the amount of sheet feeding may be larger or smaller than a predetermined one or the sheet may float due to the lack of a predetermined tension acting on the sheet.
Likewise, at the start of sheet feeding, as shown in FIG. 43B, an area of the printing medium 2 between the location of the front end thereof at the time when the medium reaches the downstream roller 3B and a location on the medium immediately below the back end of the printing width of the print head 1 as observed at that time (this area is hereinafter referred to as a “front end area”) is fed only by the upstream roller 3A. Then, the front end area of the medium faces the print head 1 while being fed. Phenomenon as described above also may occur in the case of printing on this area is performed.
The size of the front or back end area is essentially determined by the distance between the upstream roller and the downstream roller and by the printing width (in the ink jet method, the length of an array of ejection ports) of the print head arranged between the two pairs of rollers. The distance between the two pairs of rollers tends to be reduced due to demands for a reduction of an apparatus size and improving of accuracy in feeding the printing media. Thus, the size of the back or front end area is dominated by the length of the array of ejection ports of the print head. On the other hand, the printing width of the print head, that is, the length of the array of ejection ports, now tends to be increased in order to increase a printing speed. In this case, the size of the back or front end area must also be increased, as is apparent from FIGS. 43A and 43B.
The increase in the size of the back or front end area corresponds to an increase in a possibility of performing printing on this area or in an amount of printing on this area. Correspondingly, printing is performed in a condition that the printing medium is not so accurately fed.
Further, when an amount of feeding sheet at one feeding operation increases due to increase in the length of the printing width, a correspondingly significant error in the sheet feeding is likely to occur in the area of the sheet which cannot be transported so accurately. This also applies to, for example, multi-pass printing, in which the amount of sheet fed at one feeding operation is smaller than the printing width of the print head.
To solve these problems, for example, Japanese Patent Application Laid-open No. 11-291506 describes a reduction of a number of nozzles used in the above described back end area, that is, a reduction of the printing width for the purpose of reducing the amount of sheet fed at one feeding operation, thus lessening a feeding error. This publication essentially discloses the invention of a printing method of increasing resolution on a basis of interlacing to restrain the back end area from being fed inaccurately. It describes not only the reduction of the feeding amount as described above but also an increase in a size of a dot for the purpose of making the feeding error unrecognized as well as performing printing of a raster, an array of pixels extending in a scanning direction, using two different nozzles.
The invention described in the above publication, however, cannot properly deal with a phenomenon known as “kicking” the printing medium and which occurs during feeding of the back end area to cause a significant feeding error. That is, the kicking of the sheet is a phenomenon in which when the sheet is disengaged from the upstream roller and its pinch roller, the feeding force of the downstream roller or the like causes the sheet to be fed at a larger amount than normal feeding of the medium. The technique of simply reducing the feeding amount as described in the above publication may cause a more uneven print density, such as a “stripe” to appear. Further, it is difficult for the interlacing technique described in the above publication to control the sheet feeding so as to, for example, extremely reduce the feeding amount in a particular feeding operation correspondingly to the large amount of error that may occur in such particular feeding operation. For the interlacing technique, the most important point is to feed the sheet so as to accurately form dots at locations obtained by dividing nozzle pitch, and it is thus difficult to significantly change the amount of feeding in a particular feeding operation.
Another problem that may occur in the back or front end area is as follows. A phenomenon in which when the sheet is disengaged from one of the pairs of rollers, it may float due to a lack of an appropriate tension acting on the sheet (this phenomenon is hereinafter also referred to as “sheet floating”) and an error in dot forming locations, which is similar to the above-described error due to the low feeding accuracy, may occur. Further, the dots may be deviated not only in the feeding direction but also in a direction other than the feeding direction, for example, a scanning direction, because the lack of an appropriate tension acting on the sheet may cause the sheet to be curled or cause it to be warped to form projections and recesses when it comes into contact with ribs located under a sheet feeding passage. Such a dot deviation cannot be easily prevented by reducing the number of nozzles used for printing and thus reducing the feeding amount as described in the above publication, and may degrade the printed image in the back or front end area of the sheet as with the feeding error.